The present invention is concerned with isocyanates of the formula ##SPC2##
Wherein X represents fluorine or chlorine and a process for their manufacture.
The new isocyanates of formula (I) may be manufactured by methods which are in themselves known.
In principle, two general methods are available:
A. An isocyanate of the formula ##SPC3##
Can be reacted with a brominating agent, or (B) an isocyanate group can be produced in a compound of formula ##SPC4##
Wherein X represents a fluorine or chlorine atom and Y is a nitrogen-containing group that can be converted into an isocyanate group.
If route (A) is to be followed, the bromination is preferably carried out in a solvent or diluent. As the solvent or diluent there may, for example, be used aliphatic or aromatic hydrocarbons.
Of the aliphatic hydrocarbons the halogenated hydrocarbons for example ethylene dichloride or dichlorethylene, dichlorethane, tetrachlorethylene, tetrachlorethane, chloroform and carbon tetrachloride have proved to be especially suitable. Of the aromatic hydrocarbons those which are not susceptible to bromination, or are susceptible to bromination only with difficulty, are preferred, for example perhalogenated benzenes or naphthalenes and nitrobenzene.
As brominating agents there may be used, for example, bromine itself, complexes of bromine, for example, pyridine hydrobromide dibromide, aluminium bromide/bromine complexes, and organic compounds which easily release bromine, for example N-bromosuccinimide or N-bromophthalimide. Bromination with molecular bromine at a temperature within the range of from -20.degree.C to 150.degree.C, is the most simple process. Halogenation catalysts, for example, iodine or iron accelerate the reaction. The rate of reaction may be increased by the presence in the reaction mixture of a dimerised or trimerised isocyanate or urea or a symmetrical urea.
If route (B) is to be followed, then the so-called "synthetic reaction" can be used, such as for example the phosgenation of 3-chloro-4-bromaniline or 3-fluoro-4-bromaniline or of its hydrochloride. The phosgenation is preferably carried out in a solvent or diluent, for example in toluene, xylene, cumene, chlorobenzene, dichlorobenzene or trichlorobenzene. As a rule elevated temperatures are used, for example those within the range of from 110.degree.C to 200.degree.C. It is of course also possible to phosgenate in the gas phase, in which case the free amine is used in the vapour state.
The isocyanates in question can also be obtained from the corresponding N-halogenamides by the Hofmann reaction or from the corresponding acid azides by the Curtius reaction.
The isocyanates can also be obtained by so-called "reverse decomposition" reactions. Thus, for example, the corresponding isocyanates may be obtained from ureas, for example, the symmetrical ureas, by the action of phosgene.
It is also possible to manufacture the isocyanates by reacting the corresponding phenyldiazonium salts with alkali cyanides in the presence of copper powder.
The desired isocyanates may also be manufactured by means of the Lossen rearrangement of appropriately substituted hydroxamic acids. The usual reagents such as acid anhydrides, preferably, acetic anhydride, phosgene or thionyl chloride may be used as reagents for eliminating water.
The new isocyanates of formula I are above all valuable intermediates for the manufacture of herbicidally active ureas. Especially valuable selective herbicides are obtained when the isocyanates of formula I are reacted according to a known method with a N,N-di-lower alkyl amine. The lower alkyl radicals of the amines contain 1 to 4 carbon atoms each and are preferably methyl groups. Hence, specific compounds are inter alia, N-(3-fluoro-4-bromophenyl)-N',N'-dimethyl urea and N-(3-chloro-4-bromo-phenyl)-N',N'-dimethyl urea; they are prepared by reacting the relevant isocyanate with dimethylamine. Such ureas show an outstanding selective herbicidal activity in cereals, maize, rice, corn, tomatoes, potatoes, soya, cotton, carrots and others. Further possible applications of the compound of formula I are in the textile finishing field.